Spalling uniaxial strength of Al2O3 at high strain rates

In this article research into the uniaxial tensile strength of Al2O3 monolithic ceramic is presented. The experimental procedure of the spalling of long bars is investigated from different approaches. This method is used to obtain the tensile strength at high strain rates under uniaxial conditions. Different methodologies proposed by several authors are used to obtain the tensile strength. The hypotheses needed for the experimental set-up are also checked, and the requirements of the set-up and the variables are also studied by means of numerical simulations. The research shows that the shape of the projectile is crucial to achieve successfully tests results. An experimental campaign has been carried out including high speed video and a digital image correlation system to obtain the tensile strength of alumina. Finally, a comparison of the test results provided by three different methods proposed by different authors is presented. The tensile strength obtained from the three such methods on the same specimens provides contrasting results. Mean values vary from one method to another but the trends are similar for two of the methods. The third method gives less scatter, though the mean values obtained are lower and do not follow the same trend as the other methods for the different specimens

3D meso-scale modelling of concrete material in spall tests

Tensile strength is one of the key factors of concrete material that need be accurately defined in analysis of concrete structures subjected to high-speed impact loads. Dynamic tensile strength of concrete material is usually obtained by conducting laboratory tests such as direct tensile test, Brazilian splitting test and spall test. Concrete is a heterogeneous material with different components, but is conventionally assumed to be homogeneous, i.e., cement mortar only, in most previous experimental or numerical studies. The aggregates in concrete material are usually neglected owing to testing limitation and numerical simplification. It has been well acknowledged that neglecting coarse aggregates might not necessarily give accurate concrete dynamic material properties. In the present study, a 3D meso-scale model of concrete specimen with consideration of cement mortar and aggregates is developed to simulate spall tests and investigate the behaviour of concrete material under high strain rate. The commercial software LS-DYNA is used to perform the numerical simulations of spall tests. The mesh size sensitivity is examined by conducting mesh convergence tests. The reliability of the numerical model in simulating the spall tests is verified by comparing the numerical results with the experimental data from the literature. The influence of coarse aggregates on the experimental test results is studied. The wave attenuation in concrete specimen is analysed, and empirical equations are proposed for quick assessment of the test data to determine the true dynamic tensile strength of concrete material. The contributions of aggregates to dynamic strength in spall tests are quantified for modifying the test results based on mortar material in the literature

Shockless spalling damage of alumina ceramic

Ceramic materials are commonly used to build multi-layer armour. However reliable test data is needed to identify correctly models and to be able to perform accurate numerical simulation of the dynamic response of armour systems. In this work, isentropic loading waves have been applied to alumina samples to induce spalling damage. The technique employed allows assessing carefully the strain-rate at failure and the dynamic strength. Moreover, specimens have been recovered and analysed using SEM. In a damaged but unbroken specimen, interactions between cracks has been highlighted illustrating the fragmentation process

Analysis and modelling of the cohesion strength of concrete at high strain-rates

International audienceWith the exponential increase of computational power, numerical simulations are more and more used to model the response of concrete structures subjected to dynamic loadings such as detonation near a concrete structural element or projectile-impact. Such loadings lead to intense damage modes resulting from high strain-rate tensile loadings in the concrete structure. However, the modelling of the post-peak tensile response of concrete still remains difficult due to the lack of experimental data at high strain-rates. This work aims at improving the modelling of the softening behaviour of concrete based on the following statement: despite the propagation of unstable cracks in the tested specimen cohesion strength exists in the vicinity of triggered cracks and is driving the whole softening behaviour of concrete. This statement is justified in the present work by means of experiments and Monte-Carlo calculations: firstly, concrete samples have been subjected to a dynamic tensile loading by means of spalling experiments. Several specimens have been recovered in a damaged but unbroken state and have been subsequently loaded in quasi-static tensile experiments to characterise the residual strength and damage level in the sample. In addition, Monte-Carlo simulations have been conducted to clarify the possible influence of cohesion strength in the vicinity of cracks. Finally, the DFH (Denoual-Forquin-Hild) anisotropic damage model has been adapted to take into account the cohesion strength in the damaged zone and to describe the softening behaviour of concrete. Numerical simulations of experiments conducted on dry and saturated samples at different levels of loading-rate illustrate the new capability of the mode

Shock characterization of an ultra-high strength concrete

Nowadays, the design of protective structures may imply ultra-high performance concretes. These materials present a compressive strength 5 times higher than standard concretes. However, few reliable data on the shock response of such materials are available in the literature. Thus, a characterization of an ultra-high strength concrete has been conducted by means of hydrostatic and triaxial tests in the quasi-static regime, and plate impact experiments for shock response. Data have been gathered up to 6 GPa and a simple modelling approach has been applied to get a reliable representation of the shock compression of this concrete

Shockless Characterization of Ceramics

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Influence of aggregate size and free water on the dynamic behaviour of concrete subjected to impact loading

Concrete is a material widely used in civil engineering. Thus the knowledge of its mechanical behaviour is a major safety issue to evaluate the ability of a structure to resist to an intense dynamic loading. In this study, two experimental techniques have been applied to a micro-concrete and a common concrete to assess the influence of the aggregate size on the dynamic response. First, spalling tests on dry and wet specimens have been performed to characterize the tensile strength of concrete at strain rates in the range 30 – 150/s. Then, edge-on impact tests in sarcophagus configuration have been conducted. The cracking pattern of the micro-concrete and the concrete plates in wet and dry conditions have been compared to appraise the influence of aggregate size and free water on the damaging process

Tensile strength of dried mortar over a wide range of strain rate

To investigate the tensile strength of a dried mortar over a wide range of strain-rate (1e-5 s−1 – 5e3 s−1), quasi-static tests and dynamic tests have been conducted on dry specimens. The device used for the quasi-static experiments is a hydraulic press armed with ball-and-socket joints on both sides of the loading platens between which the mortar specimen is glued. In parallel, based on the spalling technique, a Hopkinson bar device has been used for intermediate loading rates (range of strain-rate from 40 s−1 to 140 s−1). Complementary, based on pulsed power technology, spall experiments have been performed on the mortar with the GEPI device at Centre d'Etudes de Gramat to reach the highest strain rates. The experimental results show a significant increase of strength with strain-rate over the range of loading rate considered

Dynamic fragmentation of an alumina ceramic subjected to shockless spalling: An experimental and numerical study

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